This competing continuation proposal aims to extend the current studies of a grammatical marker of Specific Language Impairment (SLI) in young children. Current cross-sectional and longitudinal studies provide strong support for a grammatical phenotype of SLI characterized by an extended period of acquisition of tense-related grammatical morphemes. Unresolved issues include how long this period persists in affected children, the grammatical symptomology in adolescents and adults, relationships among the grammatical marker and other linguistic and cognitive indices, and whether the grammatical marker is an inherited condition. In order to study these questions, we propose the following specific aims: (1) To identify grammatical markers for the condition of SLI for children in the adolescent age range. This would be the first documentation of detailed morphosyntactic abilities in this age range for affected children. (2) To describe the full developmental trajectory of the grammatical marker and the transition from a child grammar to an adult grammar. Recruitment and study of additional participants would generate descriptive evidence and growth curves over the period from 3 years to 19 years, comprising the most detailed and extended picture of morphosyntactic development available to date. (3) To develop and evaluate multivariate models of the grammatical marker, associated linguistic and cognitive abilities and change over time. This would provide unprecedented formal evaluation of the interrelationships between the grammatical marker and other variables over time. 4) To investigate the genetics of SLI, by assessing the language competencies of the nuclear family members of SLI probands, identifying affected family members of the probands, and initiating molecular genetic studies of the families. The aim is to increase the number of families with direct assessment and DNA samples to provide suitable power for linkage and association analyses with dichotomous and quantitative phenotypes. The research objectives outlined above will take advantage of a well-documented clinical longitudinal sample of interest because these children continue to show affectedness at early adolescence, and their families show an elevated rate of affectedness via direct assessment. The study outcomes will be immediately relevant for clinical methods of identification of affected individuals who are unlikely to "outgrow" early childhood language impairments. The outcomes will contribute advances in our understanding of individual variation in morphosyntactic competencies across the life span, and etiological factors in the condition of SLI.